It's just coming out of winter here in Perth, Western Australia. I have 6.6kw solar power and also a 15 year old solar hot water system with thermostat controlled electric booster (so not ancient, but also not the most advanced tech). On cold and cloudy days the booster turns on and draws far more power than my panels are generating at that moment, but even on nice days it turns on over night when there is no solar power. I saw on one particularly bad day that it turned on 3 times in 24h - 2x when dark - and drew over 1.5kWh each time, and I only used the hot water once in the morning. So I turned the booster off a month or 2 ago (can't remember when exactly, but yes it meant some days I couldn't have a hot shower), and on my recent 2-month power bill I had used 215kWh less than last year! (I also replaced my plasma TV with an OLED in that time, so that might have been responsible for a portion.) So whilst I get free hot water for ~8 months of the year, the other 4 months it uses a ton of electricity.

My sister recently got a new hot water system and I showed her some data that showed heat pump ones were better, so that's what she got. The data showed that whilst they use electricity during the summer, they use less during winter, and they end up ahead. But actually we can see on her power bill that when it kicks in on bad days, it too can overwhelm her solar generation at that moment.

Ultimately I think we need smarter connected systems to get the best efficiency. The booster in my solar hot water is just a basic element like a kettle. Surely it could be "throttled" (supplied with less watts) or cycled during the day so that it only heats when there is solar power available? If I have to have a slightly-cooler-than-hot shower first thing in the morning because I didn't want it boosting at 4am, then so be it.